This invention relates to ship hull designs, and in particular to hulls made of the sinusoidal water-line variety, an example of which is described in European Patent 0 134 767 B1, issued to Ramde, incorporated herein by reference. Hereinafter, such a hull may be referred to as a "Ramform" hull. A Ramform hull is characterized as having squarely cut off, approximately sinusoidal waterlines around the design waterline. These features are illustrated in FIG. 1 which shows a top plan view of a hull made according to an embodiment of the invention.
The waterlines appear approximately sinusoidal, because, as shown in the top view of FIG. 1, each side of the hull appears to be approximately one half of a wave length of a sign wave.
FIG. 1 also shows that the sinusoidal waterlines appear squarely cut off at the stern. In other words, the water line at the stern would form a line normal, or "square," to the center line of the ship which intersects, or "cuts off," the sinusoidal waterlines. Such a hull, in comparison to conventional hull designs makes it possible to improve a vessel's deadweight tonnage transverse stability, navigational and sailing properties and to reduce stresses on the hull beam whether the vessel is sailing in quiet water or into the waves.
As explained in the Ramde patent, at given main dimensions of length, breadth and depth to the design waterline, conventional hull configurations can obtain greater dead-weight tonnage by increasing the fullness of the underwater portion of the hull, thereby increasing the total displacement. To improve the transverse stability of a conventionally formed hull, expressed as a higher initial metacenter, the breadth of the hull can be increased to obtain a greater moment on inertia at the waterline, optionally also raising the volumetric center of gravity of the underwater hull.
However, changes of this nature (increasing displacement and beam), as demanded for transverse stability and speed increase, will eventually result in an unacceptable increase in a conventional vessel's resistance to propulsion in quiet waters as well as in heavy waves.
To improve the seagoing properties of a conventional hull configuration, expressed as the vessel's angular movements about a transverse axis (pitching), vertical movements (heave), accelerations and the amount of increase in propulsion resistance compared to the resistance in quiet seas, one seeks to alter the vessel's natural frequency of pitching and heaving so that this frequency, insofar as possible, does not coincide with the frequency of the wave lengths which the vessel encounters.
In the case of conventional hull designs, structural alterations result in only slight improvements in the seagoing properties of the vessel, and extreme pitching and heaving movements and a great increase in the resistance to propulsion will occur when the ship is sailing into the waves when the prevailing wave length is approximately equal in the ship's length at the waterline.
Depending on the type vessel and its rate of speed, such synchronous movements always make it necessary for a conventional ship to reduce speed or alter course in relation to the waves, thereby altering the cycles of encounter with the waves so that the wave period does not coincide with the natural frequency of the vessel's pitching and heaving.
The earlier Ramde patent taught the use of certain relationships which have been found to be non-optimal. Also, other differences have been found that make substantial improvements in the performance of ships designed according to the earlier Ramde patent, as well as curing problems found in the use of such ships.